Thermoplastic polymers are non crosslinked polymers made by polymerization like polyethylene, by polycondensation like polyesters, or by modification of another polymer like poly-vinyl alcohol. These polymers can be processed at high temperature. Under molten form, they are extruded, molded, pressed, etc. . . . .
Thermoplastic semi-crystalline polymers generally contain domains where the polymer chains are crystalline and domains where the polymer chains are amorphous. They can be melted, they can be solubilized in a solvent. Thermoplastic semi-crystalline polymers are generally more ductile than amorphous glassy thermoplastics. They are characterized by a glass transition temperature and a melting temperature. Below the melting temperature, thermoplastic semi-crystalline polymers exhibit mechanical and thermal properties better than those of amorphous thermoplastic polymers. Above the melting temperature, thermoplastic semi-crystalline polymers flow and except for very high molecular weights, viscosity reduces abruptly near the melting point. Thermoplastic semi-crystalline polymers often exhibit better solvent resistance than amorphous thermoplastic polymers but still they are soluble in organic solvents, especially at high temperature.
A thermoplastic semi-crystalline polymer can be designed by selecting monomers and polymerization conditions well known to the skilled professional.
The skilled professional knows how to check that a polymer is semi-crystalline, notably the following methods are commonly used for this purpose: Differential Scanning calorimetry (DSC), Density measurements, X-Ray Diffractometry (DRX), Polarizing Optical Microscopy (POM), Transmission Electron Microscopy (TEM), solid state NMR, vibrational spectroscopy.
Crystallizable polymer chains means polymers able at certain conditions (temperature, pressure, annealings) to show a semi-crystalline state i.e. a coexistence of domains where the polymer chains are crystalline and domains where the polymer chains are amorphous.
The skilled professional knows the methods to produce non-crosslinked polymers by polymerizing in the absence of any crosslinker, by avoiding secondary crosslinking reactions or by maintaining crosslinking below the gel point. The skilled professional can check that a polymer is below the gel point by submitting the polymer to a solubility test. For each type of polymer, the skilled professional knows which solvent to select to perform this test.
Thermoplastic semi-crystalline polymers can be crosslinked to form a three-dimensional network. Compared to a non crosslinked polymer, the crosslinked polymer network is insoluble and more resistant to creep especially at high temperatures. A semi-crystalline polymer network can be obtained by following one of the guidelines here-under which are well-known to the skilled professional:                copolymerising or condensing bifunctional monomers known to form crystallizable polymer chains and polyfunctional (with functionality superior to 2) monomers or        forming links between crystallizable polymer chains thanks to reactive functions present on said chains, or created by external stimuli such as temperature, electromagnetic radiation, electron beam or plasma.        reacting crystallizable polymer chains with a crosslinking agent such as oxygen, peroxides, sulfur.        modifying parts of a crystallizable polymer by simultaneous or sequencial cleavage reactions and crosslinking reactions.        
Once crosslinked beyond the gel point by these methods, the polymer becomes insoluble but it is no longer a thermoplastic. Above the melting temperature, the polymer does not flow or relax stresses.
The skilled professional knows how to check that he has obtained a semi-crystalline polymer network:
Crystallinity can be confirmed by any of the above-mentioned methods. It can be checked that the polymer is beyond the gel point (i.e. a network has been formed) by placing the polymer network in a solvent known to dissolve non-crosslinked polymers of the same nature. If the polymer swells instead of dissolving, the skilled professional knows that a network has been formed.
The skilled professional can refer to the following handbook to select crystallizable polymer sequences: D. W. van Krevelen Properties of Polymers Elsevier, Amsterdam 1990, J. Brandrup, E. H. Immergut Polymer Handbook Wiley Interscience New York 1989. The skilled professional can refer to the following manuals to perform any of these synthesis or test steps: P. J. Flory Principles of Polymer Chemistry Cornell University Press Ithaca-NY 1953, U. W. Gedde Polymer Physics Kluwer Academic Publishers Dordrecht 1999, L. H. Sperling Introduction to Physical Polymer Science Wiley Interscience New-York 2001, J. M. G. Cowie Polymers: Chemistry & Physics of Modern Materials Blackie Academic & Professional London 1991. However, there remains a need for better ways to control the viscosity, the plasticity and the insolubility of semi-crystalline polymers, in temperature ranges wider than those known today. The goal is to have more flexibility in modes of implementation of these materials.
Some thermoreversible semi-crystalline crosslinked polymer networks have been disclosed in the past: US2004/0059060; K. Ishida et al., Macromolecules, 2010, 43, 1011-1015; K. Ishida et al., Macromolecules, 2008, 41, 4753-4757; CN1134433; J.-M. Raquez et al., Chem. Eur. J. 2011, 17, 10135-10143; US2011/015350; US2012/309895; DE10 2010 040 282; U.S. Pat. No. 8,258,254. They are based on crosslinks dissociation by temperature change, a reaction scheme which is illustrated in FIG. 3C. Most networks disclosed therein have their reversibility based on a reversible Diels-Alder reaction. When dissociated by application of heat, network connectivity is reduced, such networks are disconnected to a point below the gel point in order to permit reshaping and/or recycling.
Networks disclosed in US2011/015350 are based on metathesis and metathesis catalysts but again, the goal is to allow the decrosslinking of a polymer network. The use of catalysts for promoting exchange reactions between olefin double bonds is neither mentioned nor suggested in this document. Compositions made by crosslinking a semi-crystalline polymer disclosed in this document have an uncertain level of crosslink density. The exact composition, the molar average molecular weight and the number average molecular weight, the number of olefinic side chains and the degree of crystallinity of the polymers are not provided. Therefore it is not possible to determine if they are beyond the gel point, if the number of olefinic double bonds in enough to have an influence on flow properties and whether the composition is semi-crystalline after crosslinking Paderni K. et al., J. Mater. Sci. (2012) 47:4354-4362, discloses semi-crystalline polymers with shape memory based on alkoxysilane-terminated poly(ε-caprolactone).
J. M. Cuevas et al., Smart Materials and Structure, vol. 20, (2011), p. 1-9, discloses shape memory polymers based on polyalkenamer crosslinked semi-crystalline networks.
P. T. Knight et al., Macromolecules, 2009, 42, 6596-6605 discloses oligosesquioxane-terminated poly(lactide-co-glycolide) semi-crystalline networks with shape memory properties.
After having been submitted to a transformation under application of heat, when re-heated, such networks recover their initial shape. In such networks, crosslinks do not exchange, application of heat above the melting temperature produces deformation by melting of the crystalline fragments, and recovery of the initial shape is made possible also by the application of heat. When the network is set at a fixed temperature, no relaxation of strain or flowing of the network is observed. In such networks, there does not exist a temperature at or above which the viscosity of the polymer network composition is inferior or equal to 1011 Pa·s.
The inventors have now discovered that networks of semi-crystalline polymers incorporating exchangeable covalent bonds make it possible to obtain semi-crystalline polymers with improved properties.
This finding is surprising: indeed, one could expect that the presence of crosslinks and covalent exchangeable bonds degrades the materials properties, including mechanical properties. Crosslinking is known to reduce crystallinity of polymers. It was expected a reduction in the crystalline, or ordered, character of these polymers, and a subsequent reduction of thermal resistance. Thermal resistance is notably evaluated by measuring the Heat Distorsion Temperature (HDT) of the polymer composition, notably by the ASTM-D648 method. But, surprisingly, the inventors have found that in the presence of crosslinks and exchangeable covalent bonds the crystalline nature of the polymers is preserved and new properties such as an increase in thermal resistance and in mechanical properties can be noted. Additionally, the presence of exchangeable covalent bonds provides these polymer networks with more flexible conditions of processability.
Polymer network compositions of the invention, wherein a semi-crystalline polymer network is associated to a catalyst, are characterized by the fact that there exists a temperature T1, above which the viscosity is inferior or equal to 1011 Pa·s.
Polymer networks of the invention are characterized by a glass transition temperature Tg, and a melting temperature Tf.
The temperature T1 may be adjusted, in particular it can be adjusted well above or close to the melting temperature of the network.
These features, which are described in more detail below can be adjusted to modulate mechanical and thermal properties of the polymer network. When T1 occurs close to Tf, the system, in comparison to crosslinked semicrystalline polymers of the prior art, shows good processability above Tf while maintaining an excellent solvent resistance. When T1 occurs well above Tf, the system shows, in comparison to its non-crosslinked counterpart, better solvent resistance, better creep resistance due to viscosity higher than 1011 Pa·s between Tf and T1 and good processability above T1. In all cases, the polymer's processability is improved: the polymer networks can have more flexible and controlled modes of transformation thanks to a better control of the viscosity and plasticity of the network
In comparison to crosslinked semicrystalline polymer compositions of the prior art, polymer network compositions according to the invention also present at equivalent degree of crystallinity, decreased thermal expansion coefficient between Tg and Tf. They also present improved chemical properties: increased solvent resistance, and at equivalent degree of crystallinity, increased impermeability to gases and liquids.
The invention also relates to a method of crosslinking semi-crystalline polymers in presence of an appropriate catalyst, wherein the number of crosslinking bonds is sufficient for the polymer network to be beyond the gel point and the number of exchangeable bonds is sufficient for the network to relax stresses and/or flow when at an appropriate temperature.